Lecture 8 0 Silicon Crystal Growth Silicon Mfg

Lecture 8. 0 Silicon Crystal Growth

Silicon Mfg. - old l Produce Silicon metal bar l Zone Refining – n times – To get purity l Cut off impure end l Use pieces to fill crystallization apparatus l Grow Mono-Crystal of large size

Zone Refining 0=x-Ut, k=CS/CL Co=solute concentration in melt or of solid on first pass Co=0 x+L Cs(x)dx - o x-L k. CL(x)dx

Si-Fe Phase Diagram

Si-O Phase Diagram

Crystal Growth

Silicon Mfg. - new l Produce ultra pure Silicon cylinder l Use pieces to fill crystallization apparatus l Grow Mono-Crystal of large size

Add Dopants to Silicon Grown Melt is maintained with a given impurity concentration l Melting Point is decreased l Solid produced has a given impurity concentation l

Ultra-pure Silicon Production l Si + 3 HCl Si. HCl 3 +H 2 – fluidized bed reactor at 500 to 700 K – Condense chlorosilane, Si. HCl 3 Distillation of liquid Si. HCl 3 l Si. HCl 3+H 2 Si + 3 HCl at 1400 K l Si vapor Deposits on Si mandrel in a purged fed batch reactor heated to 700 K l Results Large diameter Si with impurities at 10 ppt or 14 -9’s pure l

12” (30 cm) Boule

Crystal Growth

Czochralski Crystal Growth Apparatus l Figure 4. Today's Czochralski growth furnace, or crystal puller, is a far more sophisticated apparatus than that built by Gordon Teal nearly 50 years ago. It is however fundamentally identical. A crystal is pulled from a feedstock of molten material by slowly withdrawing it from the melt. Czochralski pullers often possess provisions for adding to the melt during a single pull so that crystals larger than what can be obtained in a single charge of the crucible may be produced. Today crystals of a 12 -inch diameter are possible, and the industry will spend billions to adopt this new size in the coming years. This figure was taken directly from the Mitsubishi Semiconductor – website: http: //www. egg. orjp/MSIL/ english/index -e. html!

Czochralski Growing System

12” (30 cm) Boule

Crystal Growth Steps l Induce Supersaturation – Sub cooled melt – S=exp[T Hf/(RT 2)d. T] l Nucleation l Growth at different rates on each Crystal Face l Results in crystal with a particular Crystal Habit or shape

Nucleation l Free Energy – GTOT= Gv V + A l Critical Size – R*=2 A Vm/(3 v. Rg. T ln. S) l Nucleation Rate l J=(2 D/d 5)exp[- G(R*)/(Rg. T)] D=diffusion coefficient l d= molecular diameter l

Surface Nucleation Surface energy, , is replaced by cos , where is the contact angle between phases l Geometric factors changed l Units #/(cm 2 sec) l Surface Nucleation l – Limits growth of flat crystal surfaces

Crystal Growth Boundary Layer Diffusion l Surface Diffusion l Edge Diffusion l Kink Site Adsorption l l Loss of Coordination shell at each step

Crystal Growth Rate Limiting Steps Boundary Layer Diffusion l Surface Nucleation l – Mono – Poly Screw Disslocation l Edge Diffusion l Kink Site Adsorption l Loss of Coordination shell l

Screw Surface Growth

Fluxes l Boundary Layer l Surface l Edge

Mass Transfer to Rotating Crystal l Local BL-MT Flux J[mole/(cm 2 s)] = 0. 62 D 2/3(Co-Ceq) n-1/6 w 1/2 l J[mole/(cm 2 s)] = 0. 62 D 2/3 Ceq(S-1) n-1/6 w 1/2 l – Franklin, T. C. Nodimele, R. , Adenniyi, W. K. and Hunt, D. , J. Electrochemical Soc. 135, 1944 -47(1988). – Uniform, not a function of radius!! l Crystal Growth Rate due to BL-MT as Rate Determining Step

Heat Transfer to Rotating Crystal l Local BL-HT Flux J[mole/(cm 2 s)] = h(Teq-T)/ Hf l J[mole/(cm 2 s)] • = 0. 62 k -1/3 n-1/6 w 1/2 (Teq-T)/ Hf l – Franklin, T. C. Nodimele, R. , Adenniyi, W. K. and Hunt, D. , J. Electrochemical Soc. 135, 1944 -47(1988). – Uniform, not a function of radius!! l Crystal Growth Rate due to BL-HT as Rate Determining Step

Crystal Habit l Equilibrium Shape – h 1/ 1=h 2/ 2=h 3/ 3 l Kinetic Shape – h 1=G 1(S)*t – h 2=G 2 (S)* t – h 3=G 3 (S)* t

Crystal Faces Flat Face l Stepped Face l Kinked Face l l Diffusion Distances to Kink sites are shorter on K &S Faces

Crystal Habit

Wafers Cut from Boule & Polished